Rifle scopes typically use an erecting image system located within the scope tube somewhere between the objective lens and the ocular lens in order to flip the image. This gives the image the correct orientation for land viewing. The erecting image system is usually contained within an erector tube that is held at one end with a gimbal or some other support that allows the tube to pivot about that end.
The opposite end of the erector tube is typically supported by an elevation screw, which is positioned at the top of the erector tube, a windage screw positioned on one side of the erector tube, and a biasing spring or spring system positioned at an angle diagonally opposite the windage and elevation screws. Together, the three supporting pieces constrain the movement and establish the position of the erector tube and allow the user to precisely adjust the position of the erector tube by adjusting the windage and elevation screws. The windage and elevation screws are used to adjust elevation and windage for points of impact change.
One of the biggest problems encountered when designing an erector tube system for a rifle scope results from the extremely tight tolerances required for accurate aiming and tracking of the adjustment system. For example, it is typical that a rifle scope will be adjusted in ¼ Minute of Angle (MOA) increments, or in 1/10 milliradian (mrad) increments. In one example of a rifle scope, a ¼ MOA of adjustment results in about 0.01 mm of movement of the erector tube. This is an extremely small measurement and illustrates how precise the mechanical components need to be in order to achieve accurate performance of the rifle scope.
In addition, rifle scopes are often subject to significant shock from the recoil of the rifle to which they are attached. This means the points of contact on the end of the erector tube (the elevation screw, windage screw, and biasing spring) must be extremely stable to ensure that the point of aim does not shift under shock load from the weapon. Accordingly, the spring force used in the biasing springs of rifle scopes is typically very high to ensure that there is no movement of the erector tube under recoil.
Conventional materials used in these components are brass or aluminum for the windage and elevation screws, brass or aluminum for the erector tube, and steel alloys for the biasing spring. Other materials have been used in these applications as well. However, the highest quality rifle scopes tend to use brass screws for windage and elevation adjustments, brass or aluminum for the erector tube, and steel or titanium for the biasing spring. Brass is prevalent because of its inherent anti-galling properties, which makes it ideal to use for adjustment screws instead of steel or other harder alloys. Galling is a condition whereby excessive friction between high spots results in localized welding with subsequent splitting and a further roughening of rubbing surfaces of one or both of two mating parts, resulting in inaccuracy.
Through extensive testing and analysis of existing rifle scopes, a significant problem with current designs was found. Specifically, the points of contact between the erector tube and the biasing spring, windage screw, and elevation screw experience deformation after a period of use. It is believed this occurs because the brass used in the windage and elevation screws is a soft material, as is the brass or aluminum used in the erector tube. Deformation of these materials occurs after prolonged exposure to the constant high stress imparted by a stiff biasing spring. The deformation of the erector tube, windage screw, and elevation screw causes inconsistency in the tracking of the rifle scope when adjusting the windage and elevation screws, thereby impairing the rifle scope's performance.
One prior art improvement to the conventional rifle scope design was to use a steel reinforcing ring (or some other very hard material, such as tungsten carbide) as a collar on the end of the erector tube for the biasing spring, windage screw, and elevation screw to contact. The hardened ring is much stronger, stiffer, and resistant to deformation, so this helps prolong the life of the system. However, this design creates a new problem because the hardened ring is a much harder surface than the brass windage and elevation screws that it presses against. Contacting the harder surface of the ring with the softer brass of the screws causes the brass end of the screws to deform over time with repeated adjustment of the screws.
It is believed the end of the screws deforms because of two actions occurring at the end of the screws: the spinning action of the screw as it is turned against the hardened ring, and the sliding action of the hardened ring against the screw tip as the opposite screw is adjusted. The amount of deformation is accentuated because the hardened ring on the end of the erector tube is cylindrical and contacts a flat surface of the screw tip. This tangential interface between the two parts means that the surface area of contact between the two surfaces is very small, essentially a line of contact at which rubbing occurs. A small surface area with a high bias spring pressure means that there is intense pressure applied to the end of the windage and elevation screws. This small line of contact is a concern in conventional scopes not using a hard ring as well.
Therefore, a need exists for a new and improved rifle scope with friction reducing element that prevents deformation of the windage and elevation screws. In this regard, the various embodiments of the present invention substantially fulfill at least some of these needs. In this respect, the rifle scope with friction reducing element according to the present invention substantially departs from the conventional concepts and designs of the prior art, and in doing so provides an apparatus primarily developed for the purpose of prevents deformation of the windage and elevation screws.